1. Field of the Invention
This invention relates to optical recording media capable of recording and reading information by irradiation of a light beam.
2. Description of the Prior Art
In conventional optical recording media, the characteristics of the recording layer become degraded during storage and transportation, because the recording layer is susceptible to oxidative corrosion by moisture and oxygen in air. And in the case of rewritable optical recording media, the recording layer becomes unusable because its recording and erasing characteristics are degraded after a number of recording and erasing cycles. Therefore, it has been customary to form, by the well-known thin film deposition method, a dielectric layer by use of dielectric compounds without oxygen such as aluminum nitrides, silicon nitrides, MgF.sub.2, ZnS, AlF.sub.3, etc, as disclosed in Canadian Patent No. 1,209,698 (corresponding to Japanese patent application Kokai publication No. 59-110052) and in Japanese patent application Kokai publication No. 60-131659, or oxides such as SiO.sub.2, SiO, Al.sub.2 O.sub.3, TiO.sub.2, etc, as disclosed in Japanese patent application Kokai publication No. 58-215744 or a mixture of chalcogen compounds such as ZnS and oxides such as SiO.sub.2 as disclosed in U.S. Pat. No. 4,847,132 (corresponding to Japanese patent application Kokai publication No. 63-103453) as the dielectric layer for protecting the recording layer.
The dielectric layer must have the function of protecting the recording layer during storage of the optical recording media.
And also, it must possess excellent mechanical characteristics and heat stability, because the dielectric layer provided on the optical recording medium is exposed to thermal and mechanical stresses at the time of recording and erasing.
At the time of recording, erasing and reading, on the other hand, the optical recording medium as the integral assembly of essential constituents such as a substrate, a recording layer, a dielectric layer, and the like, must absorb efficiently an irradiated beam of light and must reflect it suitably. Therefore, it is necessary that thickness of each layer composed of the optical recording medium should be arranged taking into consideration optical requirements. From the aspect of this optical design, the optical constants of the dielectric layer such as refractive index and extinction coefficient and its film thickness are important factors.
The important properties of the dielectric layer are those which satisfy the recording layer protective characteristics described above simultaneously with the thermal and mechanical requirements and optical requirements. The dielectric layers in accordance with the prior art technique are not always the best selection in these respects.
If the dielectric is an oxide or nitride, it cannot sufficiently shield oxygen and moisture and the characteristics of the recording layer become deteriorated by them. Moreover, because the dielectric layer itself is likely to peel or crack during storage especially in a high temperature and high humidity environment, the prevention of degradation of the characteristics of the recording layer is not sufficiently high. Further, diffusion of oxygen in the oxide or nitrogen in the nitride into the recording layer with the recording and erasing cycles is likely to invite degradation of the recording or erasing characteristics.
When MgF.sub.2 or ZnS is used either alone or substantially a single substance, cracks are likely to occur in the case of MgF.sub.2 and peel is likely to occur in the case of ZnS in the high temperature and high humidity environment. Accordingly, the recording layer protective characteristics are not sufficiently high, either.
In the case of the dielectric layer using the mixture of a chalcogen compound such as ZnS, and an oxide such as SiO.sub.2, the intrinsic stress occurring in the films of both ZnS and SiO.sub.2 functions as compressive stress if the films are formed by vacuum deposition. Therefore, this combination cannot relax the stress and the dielectric layer sometimes peels. Furthermore, oxygen in the oxide diffuses into the recording layer with the recording and erasing cycles and invites degradation of the recording and erasing characteristics.